An Architect speaking:
First, is this computer to stay at the office or at your friend's home? Until clarified, I´ll assume this is the computer that will be at home.
Second, are you sure you want to do CAD work from a remote location? I've done 3D work like that and "frustrating" falls short to describe the experience. The main culprit is video latency. Most remote desktop solutions either work with the client-server approach where you need to have the same application on both machines and the application itself does the network talking (this is common on UNIX style environments); or at the system level, where the screen output is captured on the server like a streaming video feed, enconded, compressed, transmitted, de-compressed, and played back on the client machine.. this approach is more common on MS-Windows based systems and while been less efficient it does work with pretty much every application.
The problem is that no matter the approach, the amount of video data traveling over the network is HUGE. Factor in that when working with CAD related applications the hand-eye-video refresh relationship has to be as close as possible.. As I said, I've done it over a LAN and it was frustrating enough that now I only do that on Emergency cases... I don't even want to imagine how it would be done over a WAN or the open Internet.
Third, I have yet to find a remote desktop solution that can deal with dual monitors. On a best case scenario -without considering the administrative overhead- the network traffic is doubled and so is the demand on your connection.
OK... if you still want to go with remote CAD access, then a QuadCore is overkill for the client machine.. the stay-at-the-house computer can even be one of those netbooks... all it has to do is work as a remote console. Sending over keyboard and mouse actions, and receiving a video-stream feed aren't very computer intensive tasks. In fact, the only component that will be stressed is the NIC. Not even the graphic card will be stressed.
The bottom line: Please try the remote-CAD thing on a real scenario before deciding about the new computer. If you friend find it responsive enough then the cheapest Best-Buy computer will suffice. On the other hand, if he finds the video-lag too high to endure -and my money is on that he will- then you need to built a computer powerful enough to run MicroStation on it's own, maybe using a FTP server or something to access the files on the office computer. That later case is a whole new scenario and paradigm to choose the PC components.